U.S. patent number 5,232,678 [Application Number 07/736,488] was granted by the patent office on 1993-08-03 for precipitated calcium carbonate.
This patent grant is currently assigned to ECC International Limited. Invention is credited to Ian S. Bleakley, Thomas R. Jones.
United States Patent |
5,232,678 |
Bleakley , et al. |
August 3, 1993 |
Precipitated calcium carbonate
Abstract
The calcium carbonate is prepared by: (a) slaking quicklime in
an aqueous medium; (b) carbonating and neutralizing the suspension
of slaked lime formed in step (a) using a gas comprising carbon
dioxide; and (c) separating the precipitated calcium carbonate
formed in step (b) from the aqueous medium in which it is
suspended. There is added 0.01% to 15% by weight, based on the
weight of dry calcium oxide, of a reagent having one or more active
hydrogen atoms (or a salt thereof), to the aqueous medium in which
the quicklime is slaked in step (a). The reagent is selected from
the group consisting of triethanolamine, mannitol, diethanolamine,
bicine, morpholine, tri-isopropanolamine, N-ethyl diethanolamine,
N,N-diethylethanolamine and sodium boroheptonate.
Inventors: |
Bleakley; Ian S. (St. Austell,
GB), Jones; Thomas R. (St. Austell, GB) |
Assignee: |
ECC International Limited
(GB)
|
Family
ID: |
10679780 |
Appl.
No.: |
07/736,488 |
Filed: |
July 26, 1991 |
Foreign Application Priority Data
|
|
|
|
|
Jul 27, 1990 [GB] |
|
|
9016552 |
|
Current U.S.
Class: |
423/432;
423/268 |
Current CPC
Class: |
D21H
19/385 (20130101); C09C 1/021 (20130101); C09C
3/08 (20130101); C01F 11/183 (20130101); D21H
17/675 (20130101); C01P 2006/12 (20130101); C01P
2006/60 (20130101); C01P 2002/30 (20130101); C01P
2004/54 (20130101); C01P 2002/10 (20130101) |
Current International
Class: |
C01F
11/18 (20060101); C09C 1/02 (20060101); C01F
11/00 (20060101); C09C 3/08 (20060101); D21H
17/00 (20060101); D21H 19/38 (20060101); D21H
19/00 (20060101); D21H 17/67 (20060101); C01B
031/24 () |
Field of
Search: |
;423/432,DIG.3,268 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lewis; Michael
Assistant Examiner: Hindrickson; Stuart L.
Attorney, Agent or Firm: Klauber & Jackson
Claims
We claim:
1. A process of preparing clusters of scalenohedral calcium
carbonate, which give good light scattering properties when used as
a paper filler or paper coating pigment, which process comprises
the following steps:
(a) adding to an aqueous medium, either a reagent having one or
more hydrogen atoms which dissociate on addition of said reagent to
said aqueous medium or a salt of said reagent, said reagent being
selected from the group consisting of triethanolamine, mannitol,
diethanolamine, bicine, morpholine, tri-isopropanolamine, N-ethyl
diethanolamine, N,N-diethylethanolamine and sodium
boroheptonate;
(b) thereafter slaking quicklime in said aqueous medium;
(c) carbonating the suspension of slaked lime formed in step (b) by
passing therethrough a sufficient amount of a gas comprising carbon
dioxide to cause the pH of the suspension to fall to substantially
neutral; and
(d) separating the precipitated calcium carbonate formed in step
(c) from said aqueous medium;
wherein said reagent or salt of said reagent is added to said
aqueous medium in an amount equal to 1%, by weight of dry calcium
oxide, in said quicklime.
2. A process according to claim 1, wherein the suspension obtained
in step (a) has a calcium hydroxide concentration of from 0.7 to
4M.
3. A process according to claim 1, wherein the aqueous medium in
step (c) is maintained at a temperature in the range from
30.degree. to 50.degree. C. and subjected to substantially
continuous agitation during step (a).
4. A process according to claim 1, wherein the suspension obtained
in step (a), is passed through a sieve of aperture size in the
range from 40 to 70 microns.
5. A process according to claim 1, wherein the suspension obtained
in step (a) is diluted to a concentration of not more than 15% by
weight of slaked lime and maintained at a temperature in the range
from 40.degree. to 65.degree. C.
6. A process according to claim 1, wherein said gas contains from 5
to 50% by volume of carbon dioxide, the remainder being air or
nitrogen.
7. A process according to claim 1, wherein said gas is admitted
into the suspension of slaked lime in the form of fine bubbles at a
rate of 0.02 to 0.10 moles of carbon dioxide per minute per mole of
calcium hydroxide.
8. A process according to claim 1, wherein the suspension obtained
in step (a) is agitated substantially continuously throughout the
carbonation step, by means of an impeller rotating at a peripheral
speed of at least 200 cm per second.
9. A process according to claim 1, wherein the pH of the suspension
is monitored throughout step (b).
10. A process according to claim 1, wherein the precipitated
calcium carbonate is separated from the aqueous medium in which it
is suspended by filtration.
Description
The present invention concerns a process for the preparation of
scalenohedral calcium carbonate suitable for use especially as a
filler in papermaking or as a pigment in a paper coating
composition.
BACKGROUND OF INVENTION
Since about 1920, chemically precipitated calcium carbonate has
been used as a pigment or filler in the paper industry. Various
chemical routes have been followed to precipitate the calcium
carbonate, but the most frequently used methods are based on the
double decomposition of sodium carbonate with either calcium
hydroxide or calcium chloride, or on the carbonation with carbon
dioxide gas of an aqueous suspension of calcium hydroxide ("milk of
lime"). Double decomposition processes generally employ by-products
of other chemical processes and therefore tend to yield calcium
carbonate products which contain unwanted salts. The process based
on the carbonation of milk of lime is performed in three stages;
firstly, the calcination of raw limestone to produce calcium oxide
or "quicklime"; secondly, the "slaking" of the quicklime with water
to produce an aqueous suspension of calcium hydroxide; and finally,
the carbonation of the calcium hydroxide with a gas comprising
carbon dioxide.
In order to prepare a precipitated calcium carbonate for the paper
industry, a process based upon the carbonation of milk of lime is
preferred because there is no serious problem of contamination of
the product with unwanted salts, and each of the three stages in
the production process can be controlled to adjust the properties
of the final product.
Calcium carbonate can be precipitated from aqueous solution in
three different crystal forms: the vaterite form which is
thermodynamically unstable, the calcite form which is the most
stable and the most abundant in nature, and the aragonite form
which is metastable under normal ambient conditions of temperature
and pressure, but converts to calcite at elevated temperature. The
aragonite form crystallises as long, thin needles having a
length:diameter ratio of about 10:1, but the calcite form exists in
several different shapes of which the most commonly found are the
rhombohedral shape in which the length and the diameter of the
crystals are approximately equal, and the crystals may be either
aggregated or unaggregated; and the scalenohedral shape in which
the crystals are like double, two-pointed pyramids having a
length:width ratio of about 4:1, and which are generally
unaggregated. All these forms of calcium carbonate can be prepared
by carbonation of milk of lime by suitable variation of the process
conditions.
A particularly desirable type of pigment for the paper industry is
known as a "bulking pigment". The opacity and brightness of a paper
sheet filled or coated with a mineral material depend on the
ability of the sheet to scatter light. If the pigment consists of
fine particles which are separated by small spaces or voids, the
scattering effect is generally enhanced, and is found to be at an
optimum when the width of the spaces or voids is about half the
wavelength of visible light, or about 0.25 microns. Bulking
pigments, or pigments consisting of fine particles separated by
spaces or voids of about the optimum size, are desirable in the
paper industry on account of their ability to scatter visible
light, but if the pigment consists of discrete fine particles, the
retention of these particles in a matrix of cellulosic papermaking
fibres is poor. To obtain good retention, the fine particles must
be aggregated together to form clusters of larger size.
High light scattering pigments currently available to the paper
industry include titanium dioxide, which is very effective but also
expensive, and fine kaolin particles which have been aggregated
either thermally or chemically. Pigments derived from kaolin are
also effective in scattering light, but are again expensive. Of the
various forms of calcium carbonate, the aragonite form is effective
as a high light scattering pigment but the process conditions
necessary for its production are stringent and difficult to
control. The rhombohedral form has crystals which are generally
unaggregated and which pack together too closely and do not leave
between them voids or spaces of the appropriate size. The
scalenohedral form may be produced relatively inexpensively and the
process conditions may be readily controlled to give aggregates of
fine crystals separated by spaces of substantially the optimum size
for light scattering, and is therefore the preferred form of
calcium carbonate for use as a bulking pigment in the paper
industry.
U.S. Pat. No. 2,081,112 (N. Statham & T.G. Leek) describes a
process for producing precipitated calcium carbonate by carbonating
milk of lime. It is recognised that the more violent the agitation
in the gas absorber, the finer will be the product, and the aim is
to create a fine mist of calcium hydroxide slurry in the presence
of the carbon dioxide-containing gas. The temperature in the gas
absorber is maintained at 50.degree.-60.degree. C., preferably
around 55.degree. C.
U.S. Pat. No. 2,964,382 (G.E. Hall, Jnr) concerns the production of
precipitated calcium carbonate by various chemical routes in which
calcium ions are contacted with carbonate ions in a precipitation
zone, including the carbonation of milk of lime. High shear,
intense turbulence is provided in the precipitation zone by means
of an impeller rotating at a peripheral speed of at least 1160 feet
per minute (589 cm. per second).
U.S. Pat. No. 3,320,026 (W.F. Waldeck) describes the production of
different forms of calcium carbonate including the scalenohedral
form. The calcium hydroxide is relatively coarse and contains at
least 50% by weight of particles larger than 10 microns. The
temperature in the gas absorber is maintained at less than
20.degree. C.
U.S. Pat. No. 4,018,877 (R.D.A. Woods) describes a carbonation
process in which there is added to the suspension in the gas
absorber, after the calcium carbonate primary nucleation stage and
before completion of the carbonation step, a complexing agent for
calcium ions, such as ethylenediamine tetraacetic acid (EDTA),
aminotriacetic acid, aminodiacetic acid or a hydroxy polycarboxylic
acid.
U.S. Pat. No. 4,157,379 (J. Arika et al) describes the production
of a chain-structured precipitated calcium carbonate by the
carbonation of calcium hydroxide suspended in water in the presence
of a chelating agent and a water-soluble metal salt.
U.S. Pat. No. 4,367,207 (D.B. Vanderheiden) describes a process in
which carbon dioxide-containing gas is introduced into an aqueous
calcium hydroxide slurry containing an anionic organpolyphosphonate
electrolyte to give a finely divided precipitated calcium
carbonate.
OBJECT OF THE INVENTION
It is an object of this invention to provide a method of producing
a calcium carbonate bulking pigment for the paper industry which is
at least as effective in light scattering as an aggregated kaolin
pigment but less expensive.
SUMMARY OF INVENTION
A precipitated calcium carbonate having improved light scattering
properties is prepared by a process comprising the following
steps:
(a) slaking quicklime in an aqueous medium;
(b) carbonating the suspension of slaked lime formed in step (a) by
passing therethrough sufficient of a gas comprising carbon dioxide
to cause the pH of the suspension to fall to substantially neutral
(about 7); and
(c) separating the precipitated calcium carbonate formed in step
(b) from the aqueous medium in which it is suspended.
According to the present invention there is added to the aqueous
medium in which the quicklime is slaked in step (a) a reagent
having one or more active hydrogen atoms, or a salt thereof. The
reagent is selected from the group consisting of triethanolamine,
mannitol, diethanolamine, bicine, morpholine, tri-isopropanolamine,
N-ethyl diethanolamine, N,N-diethylethanolamine and sodium
boroheptonate.
The reagent is added to the aqueous medium in which the quicklime
is slaked, rather than at a later stage, as it is thought that the
enhanced scattering coefficients obtained from precipitated calcium
carbonate produced by the above method are achieved as a result of
this earlier addition. The presence of the reagent during the
slaking operation appears to inhibit the formation of aggregates or
agglomerates of the fine particles of quicklime. The aqueous
suspension of slaked lime which is formed comprises fine, discrete
particles of slaked lime, and carbonation of a slaked lime
suspension of this type yields a precipitated calcium carbonate
having desirable light scattering properties.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferably the reagent is a polyhydric alcohol, a polyhydric
phenol, a polybasic acid, a protein or a compound of the general
formula: ##STR1## in which R1 and R2 (which may be the same or
different) are each a hydrogen atom, a hydrocarbon radical having
from 1 to 8 carbon atoms, a radical of the formula --(CH.sub.2)
p-COOM1, where p is 1-4, and M1 is hydrogen, an alkali metal or
ammonium, or a radical of the formula --(CH.sub.2)q--OX where q is
2-5 and X is hydrogen or a hydrocarbon radical having from 1 to 8
carbon atoms; and R3 is a hydrocarbon radical having from 1 to 8
carbon atoms, a radical of the formula --(CH.sub.2)p-COOM1, where p
is 1-4 and M1 is hydrogen, an alkali metal or ammonium, or a
radical of the formula --(CH.sub.2)q--OX where q is 2-5 and X is
hydrogen or a hydrocarbon radical having from 1 to 8 carbon atoms
or a radical of the formula --(CH.sub.2)r--N[
CH.sub.2)s-COOM2].sub.2, where r and s (which may be the same or
different) are each 2-5 and M2 is hydrogen, an alkali metal or
ammonium. Alternatively both R2 and R3 may be replaced by a radical
of the formula: --(CH.sub.2)t --O-- (CH.sub.2)t, where t is
2-5.
The amount of the reagent used is preferably in the range from 0.01
to 15%, preferably from 0.5 to 10% based on the weight of dry
calcium oxide.
In order to produce calcium carbonate in the scalenohedral form,
the quicklime is preferably added to sufficient of the aqueous
medium to give, on completion of step (a), a suspension having a
calcium hydroxide concentration of from 0.7M to 4M (5-30% w/v). The
temperature of the aqueous medium is preferably maintained in the
range from 30.degree. to 50.degree. C. and the aqueous medium is
preferably subjected to substantially continuous agitation during
the slaking step. The duration of the slaking step is conveniently
in the range from 15 to 30 minutes. On completion of the slaking
step the suspension is preferably poured through a sieve of
aperture size in the range from 40 to 70 microns in order to remove
unslaked lime and other undesirable impurities.
In step (b), in order to produce calcium carbonate in the
scalenohedral form, the suspension of slaked lime is preferably
diluted, if necessary, to a concentration of not more than 15% w/v
and maintained at a temperature in the range from 40.degree. to
65.degree. C. The carbonating gas preferably contains from 5% to
50% by volume of carbon dioxide, the remainder being conveniently
air or nitrogen. The carbon dioxide-containing gas is preferably
admitted into the suspension of slaked lime in the form of fine
bubbles. This may be achieved, for example, by admitting the gas
under pressure through a perforated plate gas sparger. The rate of
admission of the carbon dioxide-containing gas is preferably in the
range from 0.02 to 0.10 moles of carbon dioxide per minute per mole
of calcium hydroxide. The suspension is preferably agitated
substantially continuously throughout the carbonation step,
suitably by means of an impeller rotating at a peripheral speed of
at least 200 cm.s.sup.-1 and preferably monitored throughout the
carbonation step so that the admission of the carbon
dioxide-containing gas may be stopped when the pH has fallen to
about 7.
In step (c) the precipitated calcium carbonate is preferably
separated from the aqueous medium in which it is suspended by
filtration. The filter cake may then be thermally dried and milled
in order to provide a substantially dry, powdered product, or
alternatively the filter cake may be redispersed by means of a
dispersing agent for the calcium carbonate in order to provide a
concentrated aqueous suspension suitable for use, for example, in a
paper coating composition.
The present invention will now be described in more detail, with
reference to the following illustrative Examples.
EXAMPLE 1
A sample of quicklime prepared by calcining French limestone was
added to sufficient water at 40 degrees Celsius to give, on
completion of slaking, a slurry with a calcium hydroxide
concentration of 1M (7.4% w/v). The water also contained 1% by
weight, based on the dry weight of calcium oxide, of
triethanolamine. The mixture was stirred vigorously for 25 minutes
and was then poured through a No. 300 mesh British Standard Sieve
(nominal aperture 53 microns) to remove any undispersed
residue.
150 ml of the resulting calcium hydroxide slurry was carbonated by
passing therethrough a gas containing 25% by volume of carbon
dioxide in compressed air at a rate of 0.04 moles of carbon dioxide
per minute per mole of calcium hydroxide. The carbonation took
place in a vessel having a water jacket through which water was
circulated in order to maintain a substantially constant
temperature of 45 degrees Celsius within the reaction vessel. The
gas containing carbon dioxide was admitted at the bottom of the
reaction vessel through a perforated plate gas sparger immediately
above which was a turbine impeller rotating at 2000 rpm.
(peripheral speed 314 cm.per sec). The temperature and pH of the
suspension in the reaction vessel were monitored and the
carbonation was considered to be complete when the pH dropped to
7.0. The suspension was then filtered and the cake of precipitated
calcium carbonate was remixed with water to form a suspension
containing 30% by weight of dry calcium carbonate, which suspension
was used to measure the Kubelka-Munk scattering coefficient, S, of
the calcium carbonate by the following method:
A sheet of a synthetic plastics paper material, sold by Wiggins
Teape Paper Limited under the registered trade mark "SYNTEAPE", was
cut into a number of pieces each of size 10 cm.times.6 cm, and each
piece was weighted and tested for percentage reflectance to light
of 457 nm. wavelength when placed over a black background by means
of an Elrepho spectrophotometer to give the background reflectance
R.sub.b. The preweighed pieces of plastics paper were then coated
with different amounts of the suspension of precipitated calcium
carbonate to give coat weights in the range from 5 to 20
g.m.sup.-2. Each coating was allowed to dry in the air and the area
of dry coating on each piece of plastics paper was standardised by
placing a circular template over the coating and carefully removing
surplus coating which lay outside the periphery of the template.
Each piece of plastics paper bearing a coated area was then
reweighed, and, from the difference in weight and the dimensions of
the coated area, the coat weight X in kg.m.sup.-2 was
calculated.
Each coated area was then tested for reflectance to light of 457
nm. wavelength when the piece of plastics paper was placed (a) on a
black background (R.sub.o): and (b) on a pile of uncoated pieces of
the plastics paper (R.sub.l). Finally the reflectance to light of
457 nm. wavelength was measured for the pile of uncoated pieces
alone (r).
From these measurements the reflectance Rc of the coating alone was
calculated from the formula: ##EQU1## and the transmission T.sub.c
of the coating from the formula: ##EQU2##
From these two quantities it is possible to calculate a theoretical
value for the reflectance, R.sub.oo, of a coating layer of infinite
thickness of the same material from the formula: ##EQU3##
The Kubelka-Munk scattering coefficient S in M.sup.2.kg.sup.-1 may
now be calculated from the formula: ##EQU4##
The scattering coefficient S was plotted against the coat weight X
and the value of S for a coat weight of 8 g.m.sup.-2 was found by
interpolation. The value of S was found to be 301
m.sup.2.kg.sup.-1. The specific surface area of the calcium
carbonate measured by the B.E.T nitrogen adsorption method was
found to be 20.6 m.sup.2 g.sup.-1.
As a comparison the experiment was repeated exactly as described
above, except that no triethanolamine was added to the water in
which the quicklime was slaked. In this case the value for S at a
coat weight of 8 g m.sup.-2 was found to be 200 m.sup.2.kg.sup.-1.
The specific surface area of the calcium carbonate was found to be
9.8 m.sup.2 g.sup.-1.
EXAMPLE 2
Example 1 was repeated except that, instead of triethanolamine,
there were added to the water in which the quicklime was slaked 1%,
based on the weight of dry calcium oxide, of each of the reagents
listed in Table 1 (and in one case, as a control, with no reagent
added). In each case a sample of precipitated calcium carbonate was
prepared as described in Example 1 and the Kubelka-Munk scattering
coefficient S at a coat weight of 8 g.m.sup.-2 was measured as
described above. The results are given in Table 1:
TABLE 1 ______________________________________ Scattering
Coefficient Reagent m.sup.2 kg.sup.-1
______________________________________ Mannitol CH.sub.2 OH (CHOH)4
CH.sub.2 OH 269 Diethanolamine HN (CH.sub.2 CH.sub.2 OH).sub.2 274
Triethylamine N(C.sub.2 H.sub.5).sub.3 240 Diethylene glycol
O(CH.sub.2 CH.sub.2 OH).sub.2 245 Bicine (CH.sub.2 CH.sub.2
OH).sub.2.N.CH.sub.2 COOH 283 ##STR2## 273 Tri-isopropanolamine
N(CH.sub.2 CHOHCH.sub.3).sub.3 264 N-ethyldiethanolamine C.sub.2
H.sub.5 N(CH.sub.2 CH.sub.2 OH).sub.2 261 N,N-diethylethanolamine
(C.sub.2 H.sub.5).sub.2 N CH.sub.2 CH.sub.2 261 None 220
______________________________________
EXAMPLE 3
Example 1 was repeated except that, instead of triethanolamine,
there were added to the water in which the quicklime was slaked
various percentages by weight, based on the weight of dry calcium
oxide, of sodium boroheptonate. In one case, as a control, no
reagent was added to the water.
In each case a sample of precipitated calcium carbonate was
prepared as described in Example 1 and the Kubelka-Munk scattering
coefficient S at a coat weight of 8 g.m.sup.-2 was measured as
described above. The results are given in Table 2:
TABLE 2 ______________________________________ % by wt. of sodium
boroheptonate scattering based on wt. of dry calcium oxide
coefficient (m.sup.2 kg.sup.-1)
______________________________________ 0 240 0.2 262 0.5 280 1.1
301 1.6 276 2.7 254 ______________________________________
These results show that the scattering coefficient S reaches a
maximum when the dose of the sodium boroheptonate is about 1% by
weight, based on the dry weight of calcium oxide.
EXAMPLE 4
Example 3 was repeated except that quicklime prepared by calcining
a Belgian limestone was used. The specific surface area of the
slaked lime before carbonation was measured by the BET nitrogen
adsorption method. A sample of precipitated calcium carbonate was
prepared from each batch of the slaked lime as described in Example
1 and the Kubelka-Munk scattering coefficient S at a coat weight of
8 g.m.sup.-2 was measured as described above. The results are given
in Table 3:
TABLE 3 ______________________________________ % by wt. of sodium
Surface Scattering boroheptonate based on area coefficient wt. of
dry calcium oxide (m.sup.2.g.sup.-1) m.sup.2.Kg.sup.-1
______________________________________ 0 15.0 224 0.2 20.1 233 0.5
28.0 246 1.1 37.1 269 1.6 44.1 275 2.7 47.7 207
______________________________________
These results show that although the specific surface area of the
slaked lime continues to increase with increasing dose of the
reagent, within the range of reagent doses which was investigated,
the scattering coefficient S appears to reach a maximum at a
reagent dose within the range from about 1% to about 2% by weight,
based on the weight of dry calcium oxide.
EXAMPLE 5
The following is for comparative purposes only.
The experiment described in Example 1 was repeated except that no
reagent was added to the water in which the quicklime was slaked.
Instead there was added to the slurry of calcium hydroxide in the
carbonation reaction vessel before carbonation with the carbon
dioxide-containing gas was commenced, 1%, 10% and 40% by weight,
respectively, based on the weight of dry calcium oxide, of
triethanolamine.
In each case a sample of precipitated calcium carbonate was
prepared according to the method described in Example 1 and the
Kubelka-Munk scattering coefficient S at a coat weight of 8
g.m.sup.-2 was measured and the results are set forth in Table
4:
TABLE 4 ______________________________________ % by weight of
scattering coefficient triethanolamine (m.sup.2.kg.sup.-1)
______________________________________ 0 220 1 217 10 251 40 240
______________________________________
Comparing these results with those obtained in Example 1, it can be
seen that addition of triethanolamine at the carbonation stage,
rather than the slaking stage, even at a dose of 40% by weight,
based on the weight of dry calcium oxide, gave a very much smaller
improvement in the scattering coefficient, compared with that which
was obtained with addition of the reagent during the slaking
step.
* * * * *